In recent years, a rapidly increasing number of household electronic devices have become portable and cordless. There is currently an increasing demand for small-sized, light-weight batteries with high energy density to serve as power sources for operating those electric devices. In particular, lithium ion secondary batteries are expected to glow significantly in the future as power sources for notebook computers, mobile phones, audio-visual devices and the like since the batteries have high voltage and high energy density. Nickel-cadmium storage batteries or nickel-metal hydride storage batteries containing an alkaline aqueous electrolyte, which have been mainstream batteries, are being replaced with lithium ion secondary batteries.
As a positive electrode active material of a lithium ion secondary battery used for example has been a lithium-containing composite oxide such as LiCoO2, LiNiO2, LiMnO2 or LiMn2O4. These positive electrode active materials repeatedly expand and contract by charging/discharging. This causes destruction of a crystal structure or cracking of particles, and thus repetition of charge/discharge cycles is followed by a decrease in capacity and an increase in internal resistance. In view of such a problem considered have been attempts to stabilize the crystal structure by replacement of part of cobalt or nickel in the composite oxide with a different element.
For example, Japanese Patent Publication No. 3162437, Japanese Laid-Open Patent Publication No. Hei 5-242891 and Japanese Laid-Open Patent Publication No. Hei 6-168722 report that replacement of part of cobalt in a composite oxide with a different element such as magnesium improves a cycle characteristic and safety of a battery. While being capable of suppressing deterioration in cycle characteristic, this operation however has a problem that, when a battery comprising the aforesaid positive electrode active material is stored in a charged state at 85° C. for 3 days, for example, an amount of gas generated increases. Especially in the case of a battery comprising a low-profile-rectangular or low-profile battery case, since the strength of the battery case is lower as compared to a cylindrical battery, the battery thickness increases due to the gas generation, to the extent of getting out of a specification.
Although the cause of the increase in amount of the gas generated has not been clarified up until now, it is considered that, since the positive electrode active material, where part of cobalt has been replaced with magnesium, has high electrical conductivity and the surface thereof is active, reactivity with a non-aqueous electrolyte is increased and decomposition of the non-aqueous electrolyte is promoted.